There are a couple factors:
Until the gateway opens the circuit, your house and the grid are at the same level. If there is a sudden drop, that will impact devices (or if they are more sensitive to low voltages than the PW trip point).
The Powerwalls have a delay when switching modes from charging to discharging. So if they were charging when the grid dropped out, that would increase the time to start supplying power.
If there are super critical loads, then a small UPS will keep them happy during the switchover.
This matches my experience. The switch is as fast, or faster, than any other options that are within reach from a price & availability perspective, and most other options are way out of my price range (i.e. commercial solutions), or not available directly to consumers.
Also, when I've tested this with my Powerwalls (four of them), the results of the switch, and corresponding power "blip", seem to depend to some degree on the mode of the PW when the switch happens, but also the "depth" (not sure how else to describe this) of the momentary bounce can also be affected by the current home load. For instance, for one "off-grid" test the house was only using about 500watts, and solar was supplying the house and charging the Powerwall, and the switch to "off-grid" (manually induced at the home mains breaker) was not noticeable. For another test, I had both AC units running, and the house was pulling about 8kWs, the PW was in back-up mode and charging, and the switch over was very noticeable as I was in the garage and could hear the 200a contactor snap (fairly loudly) into off-grid mode, the garage ceiling lights flashed for a fraction of a second, but interestingly enough both AC units kept running.
So for me, as mentioned above, I do have my internet modem/routers/wifi and computer all on a UPS (which I had before the PW anyway, to protect from spikes and brown-outs) which will buffer this equipment from any potential switch-over related momentarily power "blip" - which will probably occur for any such solution powerful enough to power an entire home and it's AC while under load... as far as I'm aware.
Also worth mentioning, in some other threads there is some good info on using Eaton UPSs', can be small ones, as they handle frequency shifting better that most other brands, in some scenarios where the PWs are not configured ideally, and/or with some solar/inverter equipment. I've been wanting to get a couple more UPS to better configure my home network and computer power isolation and backup , so I'll probably be using the Eaton UPSs' as they seem more flexible and forgiving in Solar/Inverter environments where AC frequency shifting is in use to control loads and power routing (Grid/Home/PWs/etc), particularly in off-grid situations.
